In 1926 the Air Service was reorganized as a branch of the Army and became the U.S. Army Air Corps (USAAC). During this period, the USAAC began experimenting with new techniques, including air-to-air refueling and the development of the B-9 and the Martin B-10, the first all-metal monoplane bomber, and new fighters. In 1937, the B-17 Flying Fortress made its first appearance. In a spectacular feat of navigation, three B-17s intercepted the Italian passenger liner Rex[?] at sea.

In 1941, the Army Air Corps became the U.S. Army Air Forces. The USAAF reached status as a separate arm of the Army, with equal voice with the Army and Navy in 1943.

In Europe, the USAAF began daylight bombing operations, over objections of the Royal Air Force planners on the Combined Chiefs of Staff[?]. The US strategy involved flying bombers together, relying on the defensive firepower of a close formation. The tactic was only successful in part. American flyers took tremendous casualites during raids on the oil refineries of Ploesti[?], Rumania and the ball-bearing factories at Schweinfurt[?] and Regensburg, Germany. When the P-51 Mustang, with its increased range, was introduced to combat, American combat losses dropped, and operations during Big Week[?] in late winter of 1944 caused the Luftwaffe to lose experienced pilots.

In the Pacific theater, the USAAF used the B-29 Superfortress to launch attacks on the Japanese mainland from China. One of the major logisitical efforts of the war, "flying the Hump" over the Himalayas, took place. To carry both a bomb load and fuel and to bomb at high altitude through the jet stream affected the B-29's range. As soon as airbases on Saipan were captured in 1944, General Curtis LeMay changed strategy from high-level precision bombings to low-level incendiary bombings, aimed at destroying the distributed network of Japanese industrial manufacturing. Many Japanese cities suffered extensive damage. Tokyo suffered a firestorm in which over 100,000 persons died.